Pregabalin is known from U.S. Pat. No. 6,197,819 and is used for treating the peripheral neuropathic pain, epilepsy and anxiety disorders. U.S. Pat. No. 5,637,767 discloses its preparation by classic resolution of the racemate of 3-(aminomethyl)-5-methylhexanoic acid through the formation of diasteroisomeric salts with homochiral acids or bases, separation of the pair of diasteroisomeric salts by fractional crystallization or by crystallography followed by hydrolysis of the salt. However, such process provides pregabalin with low reaction yield with bad effects on the economy of the process, which limit its industrial application. U.S. Pat. No. 6,359,169 discloses its preparation through an enantioselective reaction, using a chiral auxiliary, for example the Evans' oxazolidone (4R,5S)-4-methyl-5-phenyl-2-oxazolidinone, which allows to carry out an asymmetric alkylation in order to insert the desired stereogenic center. After the asymmetric alkylation, generally carried out at cryogenic temperatures, the removal of the comparatively expensive chiral auxiliary is necessary, with a consequent further increase of the costs and of the production time US 2005/0283023 discloses the preparation of pregabalin through kinetic enzymatic resolution of a cyano-diester according to the following scheme:

The above process is commercially practicable, but has noticeable drawbacks, among them the use of pressurized hydrogen for the reduction of the nitrile and the use Nichel Raney, which is toxic and difficult to be used.
Organic Process Research & Development 1997; 1: 26-38, discloses another route of synthesis of pregabalin, which makes use of chloroform, which is carcinogenic. Furthermore the last step is carried out in the presence of bromine which is toxic and corrosive and requires special apparatus and precautions.
It has now been found an alternative process for the preparation of pregabalin which overcomes the drawbacks of the above mentioned processes. The new process makes use of inexpensive, low toxic and environmentally friendly reagents and does not requires special apparatus such as cryogenic reactors or high pressure hydrogenators. The new process has been surprisingly found more advantageous than the one disclosed in EP 1 992 609, particularly because it provides greater yields. For the reasons mentioned above the process of the present invention is more suitable for the production on industrial scale.
The particle size and the D50 were determined with the known laser light scattering technique, using a Malvern Mastersizer MS1 instrument under the following operating conditions:                300RF mm lens with 2.4 mm laser beam length;        500 mg sample dispersed in 10 ml of hexane (ACS reagent) with 1% of SPAN 85®, without pre-sonication, and with a stirring rate of 2500 rpm.        